I mean, how does one conclude something appearing mid-stream an ending (other than simply saying so)?
Different writing style. Unfortunately, I can't find the site I ran across the first time that gave a more detailed analysis, but here is some info from Edgar Goodspeed
Chapter 21 forms an epilogue later added to the completed gospel, probably when it was combined with the Gospels of Matthew, Mark, and Luke to form the great quartet of gospels which soon became the Scripture of the churches and later the nucleus of the New Testament. Verse 24 shows that the writer of the Epilogue, who must have been one of the editors of the Fourfold Gospel collection, is not identical with the author of the gospel. In the gospel the beloved disciple is an ideal figuresuch a follower of Jesus as would have seen him in his true greatness and in his larger relationships. But in the Epilogue the author, who has evidently passed away, is identified with this beloved disciple: "It is this disciple who testifies to these things and who wrote them down, and we know that his testimony is true." The new conclusion that now ends the book, verse 25, is even more appropriate as the Finis of the Fourfold Gospel: "There are many other things that Jesus did, so many in fact that if they were all written out, I do not suppose that the world itself would hold the books that would have to be written." It said, in effect, to those previously attached to one gospel or another: "Do not be surprised to find in this collection words and acts of Jesus that you never heard of before. He did more than even these four narratives contain, and if all he did were recorded, the books would fill the world."
The Epilogue is added to meet objections to the new gospel, to bring it more into harmony with its companion gospels, to commend it to their adherents, and to, enforce its message by a strong indorsement.